Solder paste, joining method using the same and joined structure

ABSTRACT

A solder paste including a metal component consisting of a first metal powder and a second metal powder having a melting point higher than that of the first metal, and a flux component. The first metal is Sn or an alloy containing Sn, the second metal is one of (1) a Cu—Mn alloy in which a ratio of Mn to the second metal is 5 to 30% by weight and (2) a Cu—Ni alloy in which a ratio of Ni to the second metal is 5 to 20% by weight, and a ratio of the second metal to the metal component is 36.9% by volume or greater.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No.13/404,395, filed Feb. 24, 2012, now U.S. Pat. No. 9,044,816, which is acontinuation of International application No. PCT/JP2010/063681, filedAug. 12, 2010, which claims priority to Japanese Patent Application No.2009-203611, filed Sep. 3, 2009, the entire contents of each of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a solder paste, a joining method usingthe same and a joined structure, and particularly to a solder paste thatis used for mounting electronic components and the like for example, anda joining method using the same and a joined structure.

BACKGROUND OF THE INVENTION

As a joining material used for mounting electronic components, solder(solder paste) is widely used.

For Sn—Pb-based solder which has been widely used, methods of bondingwith temperature hierarchy are widely applied in which high-temperaturesolder, for example Pb rich Pb-5Sn (melting point: 314 to 310° C.) orPb-10Sn (melting point: 302 to 275° C.) is used to solder at atemperature of 330 to 350° C., following by using low-temperaturesolder, for example an Sn-37Pb eutectic crystal (183° C.) to solder at atemperature equal to or lower than the melting point of thehigh-temperature solder described above, whereby bonding is establishedby soldering without melting the high-temperature solder used in thepreceding soldering.

Such bonding with temperature hierarchy is applied in, for example, atype of semiconductor devices in which a chip is die-bonded andsemiconductor devices for flip-chip bonding, and is an importanttechnique which is used in such a case where bonding is established in asemiconductor device, followed by further bonding the semiconductordevice itself to a board by soldering.

As a solder paste for use in this application, for example, a solderpaste comprising a mixture of (a) a second metal (or alloy) ballconsisting of a second metal such as Cu, Al, Au and Ag or a high-meltingpoint alloy containing those metals and (b) a first metal ballconsisting of Sn or In has been proposed (see Patent Document 1).

Patent Document 1 also discloses a joining method using a solder pasteand a method of producing electronic equipment.

When soldering using the solder paste of Patent Document 1, a solderpaste containing low-melting point metal (e.g. Sn) balls 51,high-melting point metal (e.g. Cu) balls 52 and a flux 53 asschematically shown in FIG. 3 (a) is heated and thereby reacted andafter soldering, a plurality of high-melting point metal balls 52 areconnected together via an intermetallic compound 54 formed between alow-melting point metal derived from the low-melting point metal balland a high-melting point metal derived from the high-melting point metalball as shown in FIG. 3 (b), and an object to be joined isbonded/connected (soldered) by this connected body.

In the case of the solder paste of Patent document 1, however, a solderpaste is heated in a soldering step to thereby generate an intermetalliccompound of a high-melting point metal (e.g. Cu) and a low-melting pointmetal (e.g. Sn), but a combination of Cu (high-melting point metal) andSn (low-melting point metal) has a low diffusion rate, so that Sn, alow-melting point metal, remains. A solder paste in which Sn remains maysuffer a considerable reduction in bonding strength under a hightemperature, and become unusable for some kinds of products to bejoined. Furthermore, Sn remaining in the soldering step may be molten torun off in a subsequent soldering step, thus raising a problem of lowreliability as high-temperature solder for use in bonding withtemperature hierarchy.

That is, for example, if a semiconductor device is produced through asoldering step in a process of producing a semiconductor device, andthereafter the semiconductor device is mounted on a board by a method ofreflow soldering, Sn remaining in the soldering step in the process ofproducing a semiconductor device may be molten to run off in the reflowsoldering step.

For forming a low-melting point metal fully into an intermetalliccompound so that Sn does not remain, heating at a high temperature andfor a long time is required in the soldering step, but it is actuallypractically impossible in view of productivity.

Patent Document 1: Japanese Patent Application Laid-Open No. 2002-254194

SUMMARY OF THE INVENTION

The present invention solves the problem described above, and an objectthereof is to provide a solder paste having a first metal and a secondmetal whose diffusibility in a soldering step is so good that ahigh-melting point intermetallic compound is generated at a lowtemperature and in short time, leaving almost none of a low-meltingpoint components after soldering and having excellent strength in hightemperature, and a joining method and a joined structure with highbonding reliability using the same.

For solving the problem described above, the solder paste of the presentinvention is a solder paste comprising a metal component consisting of afirst metal and a second metal having a melting point higher than thatof the first metal, and a flux component, wherein the first metal is oneof Sn and an alloy containing Sn, and the second metal is one of a metaland alloy which forms an intermetallic compound showing a melting pointof 310° C. or higher with the first metal and has a lattice constantdifference, i.e. a difference in between the lattice constant of theintermetallic compound and the lattice constant of the second metalcomponent, of 50% or greater.

The ratio of the second metal to the metal component is preferably 30%by volume or greater.

The first metal is preferably one of Sn alone and an alloy containing atleast one material selected from the group consisting of Cu, Ni, Ag, Au,Sb, Zn, Bi, In, Ge, Al, Co, Mn, Fe, Cr, Mg, Mn, Pd, Si, Sr, Te and P andSn.

The second metal is preferably one of a Cu—Mn alloy and a Cu—Ni alloy.

The second metal is preferably one of a Cu—Mn alloy in which the ratioof Mn to the second metal is 10 to 15% by weight and a Cu—Ni alloy inwhich the ratio of Ni to the second metal is 10 to 15% by weight.

The second metal preferably has a specific surface area of 0.05 m²·g⁻¹or greater.

At least a portion of the first metal is preferably coated on thecircumference of the second metal.

The flux preferably contains at least one material selected from thegroup consisting of: (a) at least one rosin selected from a rosin groupconsisting of a rosin, a polymerized rosin, a WW (water white) rosin anda hydrogenated rosin; (b) a rosin-based resin containing a derivative ofat least one material selected from the rosin group; and (c) a pastymatter obtained by dissolving a solid component such as a thixotropicagent such as hardened castor oil and aliphatic amide or an activatorsuch as an organic acid and a halide acid salt of amine with at leastone solvent selected from the group consisting of ethylene glycolmonobutyl ether, diethylene glycol monoethyl ether and diethylene glycolmonobutyl ether.

The flux preferably contains one of at least one material selected fromthe thermosetting resin group consisting of an epoxy resin, a phenolresin, a polyimide resin, a silicon resin, a derivative of the siliconresin, and an acryl resin, and at least one material selected from thethermoplastic resin group consisting of a polyamide resin, a polystyreneresin, a polymethacryl resin, a polycarbonate resin and a celluloseresin.

The joining method of the present invention is a method of joining anobject to be joined using a solder paste, wherein using the solder pasteaccording to any of claims 1 to 9, all of the first metal constitutingthe solder paste is formed into an intermetallic compound with thesecond metal constituting the solder paste by heating to join the objectto be joined.

The joined structure of the present invention is a joined structure inwhich an object to be joined is joined using the solder paste accordingto any of claims 1 to 9, wherein a joint, through which the object to bejoined is joined, has as main components the second metal derived fromthe solder paste and an intermetallic compound containing the secondmetal and Sn, and represents 30% by volume or less of the entire metalcomponent of the first metal derived from the solder paste.

In the joined structure of the present invention, the intermetalliccompound is preferably an intermetallic compound formed between one of aCu—Mn alloy and Cu—Ni alloy, which is the second metal derived from thesolder paste, and one of Sn alone and an alloy containing at least onematerial selected from the group consisting of Cu, Ni, Ag, Au, Sb, Zn,Bi, In, Ge, Al, Co, Mn, Fe, Cr, Mg, Mn, Pd, Si, Sr, Te and P and Sn,which is the first metal derived from the solder paste.

The solder paste of the present invention is a solder paste comprising ametal component consisting of a first metal and a second metal having amelting point higher than that of the first metal, and a flux component,wherein the solder paste contains, as a first metal, one of Sn and analloy containing Sn and contains, as a second metal, one of a metal andalloy which forms with the first metal an intermetallic compound showinga melting point of 310° C. or higher and in which the lattice differencebetween itself and the intermetallic compound is 50% or greater, so thatdiffusion of the first metal and the second metal rapidly proceeds,their change into an intermetallic compound having a higher meltingpoint is facilitated and no low-melting point components remain, thusmaking it possible to solder with increased strength in hightemperature.

That is, for example, by using the solder paste of the presentinvention, when a semiconductor device is produced through a solderingstep in a process of producing a semiconductor device, and thereafterthe semiconductor device is mounted on a board by a method of reflowsoldering, the soldered part in the previous soldering step hasexcellent strength in high temperature and therefore is not remelted inthe reflow soldering step, thus making it possible to mount thesemiconductor device on the board with high reliability.

In the present invention, “lattice constant difference” is defined as avalue (%) determined by subtracting the lattice constant of the secondmetal component from the lattice constant of the intermetallic compoundand dividing the obtained value by the lattice constant of the secondmetal component, followed by multiplying an absolute value of theobtained value by 100.

That is, the lattice constant difference shows a difference between thelattice constant of an intermetallic compound newly generated at theinterface with the second metal and the lattice constant of the secondmetal, and does not consider which lattice constant is greater.

The lattice constant difference is expressed by the followingcalculation formula:Lattice constant difference={(Lattice constant of intermetalliccompound−Lattice constant of second metal)/Lattice constant of secondmetal}×100.

BRIEF EXPLANATION OF THE DRAWINGS

FIGS. 1(a) to 1(c) are views schematically showing behaviors when thesolder paste of the present invention is used to establish bonding,wherein FIG. 1(a) shows a situation before heating, FIG. 1(b) shows asituation in which heating is started and a first metal is molten, andFIG. 1(c) shows a situation in which heating is further continued andall of the first metal forms an intermetallic compound with a secondmetal.

FIG. 2 is a view showing a reflow profile when the solder paste of thepresent invention is used to mount a brass terminal on an oxygen-free Cuplate.

FIGS. 3(a) and 3(b) are views showing behaviors of solder when aconventional solder paste is used to solder, wherein FIG. 3(a) shows asituation before heating and FIG. 3(b) shows a situation aftercompletion of a soldering step.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1(a) to 1(c) are views schematically showing a behavior whensoldering is carried out using the solder paste of the presentinvention.

When a pair of electrodes 11 a and 11 b are joined using the solderpaste of the present invention as shown in FIG. 1(a), a solder paste 10is first placed between a pair of electrodes 11 a and 11 b.

Next, the soldered part is heated in this state, and when thetemperature of the solder paste 10 reaches the melting point of a firstmetal (Sn or alloy containing Sn) 1 or higher, the first metal 1 in thesolder paste 10 is molten.

Thereafter, heating further continues, and the first metal 1 forms anintermetallic compound 3 with a second metal 2 (FIG. 1 (c)). Since thesolder paste 10 of the present invention has a large lattice constantdifference between the intermetallic compound 3 generated at theinterface between the first metal 1 and the second metal 2 and thesecond metal 2 (i.e. the lattice constant difference between the secondmetal 2 and the intermetallic compound 3 is 50% or greater), theintermetallic compound is repeatedly reacted while separating anddiffusing in the molten first metal, generation of the intermetalliccompound rapidly proceeds and the content of the first metal 1 can berapidly reduced in short time sufficiently (FIG. 1(a), 1(b)). Further,by optimizing the composition ratio of the first metal 1 and the secondmetal 2, all the first metal 1 can be formed into the intermetalliccompound as shown in FIG. 1(c) (see FIG. 1(c)).

As a result, soldering with increased strength in high temperature ispossible.

By ensuring that the ratio of the second metal to the metal componentconsisting of the first metal and the second metal is 30% by volume orgreater, the ratio of remaining Sn in the soldering step can be furtherreduced to further improve heat resistance.

By using, as the first metal, Sn alone or an alloy containing at leastone material selected from the group consisting of Cu, Ni, Ag, Au, Sb,Zn, Bi, In, Ge, Al, Co, Mn, Fe, Cr, Mg, Mn, Pd, Si, Sr, Te and P and Sn,the intermetallic compound can be easily formed with the other metal(second metal), and the present invention can be made more effective.

By using, as the second metal, a Cu—Mn alloy or a Cu—Ni alloy,particularly a Cu—Mn alloy with the ratio of Mn of 10 to 15% by weightor a Cu—Ni alloy with the ratio of Ni of 10 to 15% by weight, theintermetallic compound can be easily formed with the first metal at alower temperature and in shorter time, and prevented from being moltenin a subsequent reflow step.

The second metal may contain impurities at a level in which the reactionwith the first metal is not hindered, for example, at a ratio of 1% byweight or less. Examples of the impurities include Zn, Ge, Ti, Sn, Al,Be, Sb, In, Ga, Si, Ag, Mg, La, P, Pr, Th, Zr, B, Pd, Pt, Ni and Au.

When considering joining characteristics and reactivity, theconcentration of oxygen in the first and second metals is preferably2000 ppm or less, especially preferably 10 to 1000 ppm.

By using, as the second metal, one having a specific surface area of0.05 m²·g⁻¹ or greater, the probability of contact with the first metalincreases, so that the intermetallic compound can be further easilyformed with the first metal, thus making it possible to complete meltingpoint elevation with a common reflow profile.

By coating at least a portion of the first metal powder on thecircumference of the second metal powder, the intermetallic compound canbe further easily formed between the first metal and the second metal,and the present invention can be made more effective.

For the solder paste of the present invention, various kinds ofwell-known materials including a vehicle, a solvent, a thixotropicagent, an activator or the like can be used as a flux.

Specific examples of the vehicle include rosin-based resins andsynthetic resins consisting of a rosin and a derivative such as amodified rosin obtained by modifying the rosin, or mixtures thereof.

Specific examples of the rosin-based resin consisting of a resin and aderivative such as a modified rosin obtained by modifying the rosininclude gum rosins, tall rosins, wood rosins, polymerized rosins,hydrogenated rosins, formylated rosins, rosin esters, rosin modifiedmaleic resins, rosin modified phenol resins, rosin modified alkyd resinsand various kinds of other rosin derivatives.

Specific examples of the synthetic resin consisting of a resin and aderivative such as a modified rosin obtained by modifying the rosininclude polyester resins, polyamide resins, phenoxy resins and terpenresins.

As the solvent, alcohols, ketones, esters, ethers aromatics,hydrocarbons and the like are known, and specific examples includebenzyl alcohol, ethanol, isopropyl alcohol, butanol, diethylene glycol,ethylene glycol, ethyl cellosolve, butyl cellosolve, ethyl acetate,butyl acetate, butyl benzoate, diethyl adipate, dodecane, tetracene,α-terpineol, terpineol, 2-methyl 2,4-pentanediol, 2-ethyl hexanediol,toluene, xylene, propylene glycol monophenyl ether, diethylene glycolmonohexyl ether, ethylene glycol monobutyl ether, diethylene glycolmonobutyl ether, diisobutyl adipate, hexylene glycol, cyclohexanedimethanol, 2-terpinyloxy ethanol, 2-dihydroterpinyloxy ethanol andmixtures thereof.

Specific examples of the thixotropic agent include hardened castor oil,carnauba wax, amides, hydroxy fatty acids, dibenzylidene sorbitol,bis(p-methylbenzylidene)sorbitols, bees wax, amide stearate andethylenebisamide hydroxystearate. Materials obtained by adding to theabove-mentioned substances, a fatty acid such as caprylic acid, lauricacid, myristic acid, palmitic acid, stearic acid and behenic acid, ahydroxy fatty acid such as 1,2-hydroxystearic acid, an antioxidant, asurfactant, an amine and the like as required may also be used as thethixotropic agent.

Examples of the activators include halide acid salts of amine, organichalogen compounds, organic acids, organic amines and polyvalentalcohols, and specific examples of the halide acid salt of amine includediphenylguanidine hydrobromide, diphenylguanidine hydrochloride,cyclohexylamine hydrobromide, ethylamine hydrochloride, ethylaminehydrobromide, diethylaniline hydrobromide, diethylaniline hydrochloride,triethanolamine halide acid salts, and monoethanolamine hydrobromide.

Specific examples of the organic halogen compound include chlorinatedparaffin, tetrabromoethane, dibromopropanol, 2,3-dibromo-1,4-butanediol,2,3-dibromo-2-butene-1,4-diol and tris(2,3-dibromopropyl)isocyanurate.

Specific examples of the organic acid include malonic acid, fumaricacid, glycolic acid, citric acid, malic acid, succinic acid,phenylsuccinic acid, maleic acid, salicylic acid, anthranilic acid,glutaric acid, suberic acid, adipic acid, sebacic acid, stearic acid,abietic acid, benzoic acid, trimellitic acid, pyromellitic acid anddodecanoic acid, and specific examples of the organic amine includemonoethanolamine, diethanolamine, triethanolamine, tributylamine,aniline and diethylaniline.

Examples of the polyvalent alcohol include erythritol, pyrogallol andribitol.

For the solder paste of the present invention, the action effect of thepresent invention can be reliably achieved when using a flux containingat least one material selected from the group consisting of (a) at leastone rosin selected from a rosin group consisting of a rosin, apolymerized rosin, a WW (water white) rosin and a hydrogenated rosin,(b) a rosin-based resin containing a derivative of at least one materialselected from the rosin group, and (c) a pasty matter obtained bydissolving a solid component such as a thixotropic agent such ashardened castor oil and aliphatic amide or an activator such as anorganic acid and a halide acid salt of amine with at least one solventselected from the group consisting of ethylene glycol monobutyl ether,diethylene glycol monoethyl ether and diethylene glycol monobutyl ether,including other well-known materials.

The action effect of the present invention can also be further reliablyachieved when using, as the flux, one containing at least one materialselected from the thermosetting resin group consisting of an epoxyresin, a phenol resin, a polyimide resin, a silicon resin, a derivativeof the silicon resin, and an acryl resin or at least one materialselected from the thermoplastic resin group consisting of a polyamideresin, a polystyrene resin, a polymethacryl resin, a polycarbonate resinand a cellulose resin.

In the joining method of the present invention, using the solder pasteof the present invention, a low-melting point metal constituting thesolder paste is formed into an intermetallic compound with the secondmetal constituting solder paste to join an object to be joined, so thatdiffusion of the first metal and the second metal rapidly proceeds in asoldering step, their change into an intermetallic compound having ahigher melting point is facilitated and the ratio of the first metalcomponent to the entire metal component is, for example, 30% by volumeor less, thus making it possible to solder with increased strength inhigh temperature.

Further, by optimizing the metal compounding ratio in the solder pasteand the like, the solder paste can be designed such that the first metalcomponent does not remain at all.

That is, for example, by using the solder paste of the presentinvention, when a semiconductor device is produced through a solderingstep in a process of producing a semiconductor device, and thereafterthe semiconductor device is mounted on a board by a method of reflowsoldering, the soldered part in the previous soldering step hasexcellent strength in high temperature and therefore is not remelted inthe reflow soldering step, thus making it possible to mount thesemiconductor device on the board with high reliability.

In the joined structure of the present invention, a joint, where anobject to be joined is joined, has as main components the second metalderived from the solder paste and an intermetallic compound containingthe second metal and Sn, and represents 30% by volume or less of theentire metal component of the first metal derived from the solder paste,thus making it possible to provide a joined structure with increasedstrength in high temperature.

The content of the first metal derived from the solder paste in thejoint is further preferably 3% by volume or less.

For the joined structure of the present invention, as shown in FIG.1(c), in a joint (solder) 4, where objects to be joined (electrodes) 11a and 11 b are joined, all of the first metal forms an intermetalliccompound 3 with the second metal, so that the joint 4 is constituted bythe second metal 2 and the intermetallic compound 3 and no first metal 1remains (FIG. 1(a), 1(b)), thus making it possible to achieve a joinedstructure with increased strength in high temperature.

When the intermetallic compound is an intermetallic compound formedbetween a Cu—Mn alloy or Cu—Ni alloy which is the second metal derivedfrom the solder paste and Sn alone or an alloy containing at least onematerial selected from the group consisting of Cu, Ni, Ag, Au, Sb, Zn,Bi, In, Ge, Al, Co, Mn, Fe, Cr, Mg, Mn, Pd, Si, Sr, Te and P and Sn,which is the first metal derived from the solder paste, a joinedstructure in which almost no first metal component remains and which hasincreased strength in high temperature can be provided more reliably.

Examples are shown below to describe features of the present inventionfurther in detail.

Example 1

In this example 1, a solder paste was prepared by mixing a first metalpowder, a second metal powder and a flux.

The compounding ratio of the first metal powder and the second metalpowder was adjusted so that the volume ratio of the first metalpowder/second metal powder was 60/40 (i.e. second metal: 40% by volume).

As the first metal powder, Sn-3Ag-0.5Cu, Sn, Sn-3.5Ag, Sn-0.75Cu,Sn-58Bi, Sn-0.7Cu-0.05Ni, Sn-5Sb, Sn-2Ag-0.5Cu-2Bi, Sn-57Bi-1Ag,Sn-3.5Ag-0.5Bi-8In, Sn-9Zn and Sn-8Zn-3Bi were used as shown in Table 1.The average particle size of the first metal powder was 25 μm.

In writing of each material described above, for example, the digit(3.5) of “Sn-3.5Ag” represents a value in % by weight of a componentconcerned (Ag in this case), and the same applies to other materialsdescribed above and those described below.

As the second metal powder, Cu-10Ni, Cu-10Mn, Cu-12Mn-4Ni, Cu-10Mn-1P, amixed powder of equal amounts of Cu-10Ni and Cu-10Mn, Cu, and Cu-10Znwere used as shown in Table 1.

The average particle size of the second metal powder was 15 μm.

As the flux, one having a compounding ratio of rosin: 74% by weight,diethylene glycol monobutyl ether: 22% by weight, triethanolamine: 2% byweight and hydrogenated castor oil: 2% by weight was used.

For the compounding ratio of the flux, the ratio of the flux to theentire solder paste was 10% by weight.

The prepared solder paste was printed on an oxygen-free Cu plate havinga size of 10 mm×10 mm and a thickness of 0.2 mm using a metal mask. Theopening size of the metal mask was 1.5 mm×1.5 mm and the thickness was100 μm.

A brass terminal (size: 1.2 mm×1.0 mm×1.0 mm) plated with Ni and platedwith Au was mounted on the printed solder paste, followed by using areflow apparatus to establish bonding with a reflow profile shown inFIG. 2.

[Evaluation of Characteristics]

For samples prepared as described above, the bonding strength and thesolder runoff failure occurrence rate were measured to evaluate thecharacteristics.

<<Bonding Strength>>

The shear strength of the obtained joined body was measured using abonding tester and evaluated.

Measurements of the shear strength were made at a side push speed of 0.1mm·s⁻¹ at room temperature and 260° C.

Samples having a shear strength of 20 Nmm⁻² or greater were rated as ⊙(excellent) and those having a shear strength of 2 Nmm⁻² or less wererated as X (failure).

Table 1 shows the compositions of the first metal and the second metal,the lattice constant of the second metal, the compounding ratio of thefirst metal and the second metal, the type and the lattice constant ofan intermetallic compound initially generated at the surface of thesecond metal powder, the lattice constant difference between the secondmetal (Cu alloy) and the intermetallic compound and the bonding strengthof each joined body (room temperature and 260° C.). The lattice constantwas evaluated on the basis of the a axis.

<<Evaluation of Remaining Components>>

About 7 mg of the reaction product obtained was cut off and subjected todifferential scanning calorimetry (DSC measurement) under conditions ofa measurement temperature of 30° C. to 300° C., a temperature rise rateof 5° C./min, N₂ atmosphere and a reference of Al₂O₃. The amount ofremaining first metal component was quantified from the amount ofabsorbed heat in the melt heat absorption peak at the melt temperatureof the first metal component in the DSC chart obtained. Thus, the ratioof the first metal component to the entire metal component was evaluatedas a remaining first metal component rate. Samples having a remainingfirst metal component rate of 0 to 3% by volume were rated as ⊙(excellent) and those having a remaining first metal component rategreater than 30% by volume were rated as x (failure).

Table 1 shows both the remaining first metal component rate andevaluation results.

<<Solder Runoff>>

A Cu land of a printed board (Cu land size: 0.7 mm×0.4 mm) was coatedwith the solder paste (thickness of 100 μm) and a chip type ceramiccondenser having a length of 1 mm, a width of 0.5 mm and a thickness of0.5 mm was mounted on the obtained coated part.

After reflow soldering at a peak temperature of 250° C., the printedboard was sealed with an epoxy resin, left to stand in an atmospherewith a relative humidity of 85% and heated under reflow conditions at apeak temperature of 260° C. to examine a rate at which solder ran off,and the rate was evaluated as a solder runoff failure occurrence rate.

Samples having a solder runoff failure rate of 0 to 10% were rated as ⊙(excellent) and those having a solder runoff failure rate greater than50% were rated as X (failure).

Table 1 shows both the solder runoff failure occurrence rate andevaluation results.

TABLE 1 Evaluation of Intermetallic Lattice Evaluation of remainingSecond metal compound constant bonding Evaluation components EvaluationFirst component generated at difference strength of bonding Remaining ofmetal (40% by volume) interface between (room strength first metalrunoff compo- Lattice Lattice Cu alloy temperature) (260° C.) componentRunoff nent constant constant and Bonding Bonding rate failure (60% byComp- a Comp- a com- strength Eval- strength Eval- (% by Eval- rateEval- volume) osition (nm) osition (nm) pound (Nmm⁻²) uation (Nmm⁻²)uation volume) uation (%) uation Exam- Sn-3Ag- Cu-10Ni 0.357 Cu₂NiSn0.597 67 28 ⊙ 25 ⊙  0 ⊙  0 ⊙ ple 0.5Cu Cu-10Mn 0.367 Cu₂MnSn 0.617 68 28⊙ 24 ⊙  0 ⊙  0 ⊙ Sn Cu-10Mn 0.367 Cu₂MnSn 0.617 68 29 ⊙ 24 ⊙  0 ⊙  0 ⊙Sn-3.5Ag Cu-10Mn 0.367 Cu₂MnSn 0.617 68 27 ⊙ 22 ⊙  0 ⊙  0 ⊙ Sn-0.75CuCu-10Mn 0.367 Cu₂MnSn 0.617 68 27 ⊙ 24 ⊙  0 ⊙  0 ⊙ Sn-58Bi Cu-10Mn 0.367Cu₂MnSn 0.617 68 27 ⊙ 26 ⊙  0 ⊙  0 ⊙ Sn-0.7Cu- Cu-10Mn 0.367 Cu₂MnSn0.617 68 28 ⊙ 24 ⊙  0 ⊙  0 ⊙ 0.05Ni   Sn-5Sb Cu-10Mn 0.367 Cu₂MnSn 0.61768 30 ⊙ 26 ⊙  0 ⊙  0 ⊙ Sn-2Ag- Cu-10Mn 0.367 Cu₂MnSn 0.617 68 29 ⊙ 26 ⊙ 0 ⊙  0 ⊙ 0.5Cu-2Bi   Sn-57Bi- Cu-10Mn 0.367 Cu₂MnSn 0.617 68 29 ⊙ 25 ⊙ 0 ⊙  0 ⊙ 1Ag   Sn-3.5Ag- Cu-10Mn 0.367 Cu₂MnSn 0.617 68 28 ⊙ 24 ⊙  0 ⊙ 0 ⊙ 0.5Bi-8In   Sn-9Zn Cu-10Mn 0.367 Cu₂MnSn 0.617 68 27 ⊙ 25 ⊙  0 ⊙  0⊙ Sn-8Zn- Cu-10Mn 0.367 Cu₂MnSn 0.617 68 27 ⊙ 24 ⊙  0 ⊙  0 ⊙ 3Bi Sn-3Ag-Cu-12Mn- 0.367 Cu₂MnSn 0.617 68 28 ⊙ 23 ⊙  0 ⊙  0 ⊙ 4Ni 0.5Cu Cu-10Mn-0.367 Cu₂MnSn 0.617 68 29 ⊙ 23 ⊙  0 ⊙  0 ⊙ 1P Cu-10Ni 0.352 Cu₂NiSn0.597 67 28 ⊙ 25 ⊙  0 ⊙  0 ⊙ (20% by volume) Cu- 0.367 Cu₂MnSn 0.617 6810Mn(20% Compar- by volume) ative Sn-3Ag- Cu 0.361 Cu₃Sn 0.432 20 25 ⊙0.1 X 31 X 75 X Exam- 0.5Cu Cu-10Zn 0.359 Cu₃Sn 0.432 20 31 ⊙ 1.8 X 34 X70 X ple

As shown in Table 1, for the bonding strength at room temperature, bothExamples and Comparative Example showed a bonding strength of 20 Nmm⁻²or greater and were found to have a practical strength.

On the other hand, for the bonding strength at 260° C., ComparativeExample had an insufficient bonding strength of 2 Nmm⁻² or less, whileExamples retained a bonding strength of 10 Nmm⁻² or greater and werefound to have a practical strength.

For the remaining first metal component rate, Comparative Example had arate greater than 30% by volume while Examples all had a rate of 0% byvolume, and for the solder runoff failure rate, Comparative Example hada rate of 70% or greater while Examples all had a rate of 0% and werefound to have high heat resistance.

Samples of Examples were found to have comparable high heat resistanceirrespective of the type of the first metal as long as the first metalwas a solder alloy based on Sn.

Moreover, samples of Examples were found to have comparably high heatresistance as well when the second metal was a metal based on Cu—Mn(Cu-12Mn-4Ni and Cu-10Mn-1P, etc.) and the second metal powder were oftwo or more types (Cu—Mn, Cu—Ni mixed powder).

It can be considered that the reason why samples of Examples thus havehigh heat resistance is that for Examples using Cu—Mn and Cu—Ni basedalloys as the second metal, intermetallic compounds are Cu₂MnSn andCu₂NiSn, respectively, and the lattice constant difference between eachintermetallic compound and the second metal (Cu alloy) is 50% orgreater. In other words, it can be considered that this is because ifthe lattice constant difference between an intermetallic compound layergenerated and the second metal which is a base metal is large, theintermetallic compound is repeatedly reacted while separating anddiffusing in the molten first metal, and therefore formation of theintermetallic compound rapidly proceeds.

On the other hand, it can be considered that when a Cu or Cu—Zn alloy isused as the second metal as in Comparative Example, the intermetalliccompound at the bonding interface is Cu₃Sn, the lattice constantdifference between the intermetallic compound and the second metal (Cualloy) is as small as 20%, formation of the intermetallic compound doesnot efficiently proceed, and therefore high heat resistance cannot beobtained.

Example 2

A powder of Sn-3Ag-0.5Cu was prepared as the first metal powder. Theaverage particle size of the first metal powder was 25 μm.

Powders of Cu and Cu-10Mn were prepared as the second metal powder. Theaverage particle size of the second metal powder was 15 μm.

As the flux, one having a compounding ratio of rosin: 74% by weight,diethylene glycol monobutyl ether: 22% by weight, triethanolamine: 2% byweight and hydrogenated castor oil: 2% by weight was prepared.

A solder paste was prepared by mixing the above-mentioned first metalpowder, second metal powder and flux.

The compounding ratio of the first metal powder and the second metalpowder was adjusted so that the volume ratio of the first metalpowder/second metal powder was 87/13 to 57/43 (i.e. second metal powder:13 to 43% by volume).

For the compounding ratio of the flux, the ratio of the flux in theentire solder paste was 10% by weight.

For the solder paste thus prepared, the bonding strength and the solderrunoff failure occurrence rate were measured to evaluate thecharacteristics in the same manner as in Example 1.

For evaluation of the bonding strength, samples having a shear strengthof 20 Nmm⁻² or greater were rated as ⊙ (excellent), those having a shearstrength greater than 2 Nmm⁻² and less than 20 Nmm⁻² were rated as ◯(good) and those having a shear strength of 2 Nmm⁻² or less were ratedas x (failure).

For the remaining first metal component rate, samples having a rate of 0to 3% by volume were rated as ⊙ (excellent), those having a rate greaterthan 3% by volume and equal to or less than 30% by volume were rated as◯ (good) and those having a rate greater than 30% by volume were ratedas x (failure).

For the solder runoff failure rate, samples having a rate of 0 to 10%were rated as ⊙ (excellent), those having a rate greater than 10% andequal to or less than 50% were rated as ◯ (good) and those having a rategreater than 50% were rated as x (failure).

Table 2 shows the bonding strength of each joined body (roomtemperature, 260° C.), the remaining first metal component rate, thesolder runoff failure rate and the evaluation results thereof.

TABLE 2 Evaluation of Metal components in solder remaining componentspaste Evaluation of Evaluation of Remaining Evaluation of Ratio ofbonding strength bonding strength first metal runoff first metal (roomtemperature) (260° C.) component Runoff component Ratio of secondBonding Bonding rate failure Sn-3Ag-0.5Cu metal component strengthstrength (% by rate (% by volume) (% by volume) (Nmm⁻²) Evaluation(Nmm⁻²) Evaluation volume) Evaluation (%) Evaluation Example 57.1Cu-10Mn 42.9 25 ⊙ 23 ⊙ 0 ⊙ 0 ⊙ 66.7 33.3 28 ⊙ 24 ⊙ 0 ⊙ 0 ⊙ 70.0 30.0 30⊙ 26 ⊙ 0 ⊙ 0 ⊙ 72.7 27.3 31 ⊙ 16 ◯ 11 ◯ 11 ◯ 79.9 20.1 29 ⊙ 11 ◯ 16 ◯ 14◯ 84.2 15.8 27 ⊙ 9 ◯ 21 ◯ 21 ◯ 86.9 13.1 27 ⊙ 7 ◯ 26 ◯ 49 ◯ 63.1 Cu-10Ni36.9 30 ⊙ 27 ⊙ 0 ⊙ 0 ⊙ 70.0 30.0 33 ⊙ 29 ⊙ 0 ⊙ 0 ⊙ 83.7 16.3 27 ⊙ 8 ◯ 23◯ 11 ◯ Comparative 57.1 Cu 42.9 25 ⊙ 0.1 X 31 X 75 X Example 66.7 33.325 ⊙ 0.1 X 40 X 71 X 72.7 27.3 29 ⊙ 0.1 X 47 X 80 X 79.9 20.1 29 ⊙ 0.1 X53 X 79 X 84.2 15.8 30 ⊙ 0.1 X 60 X 81 X 86.9 13.1 28 ⊙ 0.1 X 74 X 85 X

As shown in Table 2, for the bonding strength at room temperature, bothExamples and Comparative Examples showed a bonding strength of 20 Nmm⁻²or greater and were found to have a practical strength.

On the other hand, for the bonding strength at 260° C., ComparativeExamples had an insufficient bonding strength of 0.1 Nmm⁻², far below 2Nmm⁻², while Examples retained a bonding strength of 7 to 26 Nmm⁻²,greater than 2 Nmm⁻², and were found to have a practical strength.Particularly, Examples showed a bonding strength of 23 Nmm⁻² or greaterand were found to have increased strength in high temperature when thesecond metal was Cu-10Mn and its ratio was 30% or greater.

For the remaining first metal component rate, Comparative Examples had arate greater than 30% by volume while Examples all had a rate of 30% byvolume or less, and Examples had a remaining first metal component rateof 0% by volume when the ratio of Cu-10Mn or Cu-10Ni, i.e. the secondmetal, was 30% by volume or greater. For the solder runoff failure rate,Comparative Examples had a rate of 70% or greater while Examples all hada rate of 50% or less, and Examples had a solder runoff failure rate of0% and were found to have high heat resistance when the ratio of Cu-10Mnor Cu-10Ni, i.e. the second metal, was 30% by volume or greater.

Example 3

A powder of Sn-3Ag-0.5Cu was prepared as the first metal powder. Theaverage particle size of the first metal powder was 25 μm.

A powder of a Cu—Mn alloy with the ratio of Mn of 5 to 30% by weight anda powder of a Cu—Ni alloy with the ratio of Ni of 5 to 20% by weightwere prepared as the second metal powder. The average particle size ofthe second metal powder was 15 μm.

As the flux, one having a compounding ratio of rosin: 74% by weight,diethylene glycol monobutyl ether: 22% by weight, triethanolamine: 2% byweight and hydrogenated castor oil: 2% by weight was prepared.

A solder paste was prepared by mixing the above-mentioned first metalpowder, second metal powder and flux.

For the compounding ratio of the flux, the ratio of the flux to theentire solder paste was 10% by weight.

The compounding ratio of the first metal powder and the second metalpowder was adjusted so that the volume ratio of the first metalpowder/second metal powder was 60/40 (i.e. second metal powder: 40% byvolume).

For the solder paste thus prepared, the bonding strength, the remainingfirst metal component rate and the solder runoff failure occurrence ratewere measured to evaluate the characteristics in the same manner as inExample 1.

Evaluation of the bonding strength and evaluation of the remaining firstmetal component rate and the solder runoff failure rate were carried outon the basis of the same criteria as in Example 2.

Table 3 shows the bonding strength of each joined body (roomtemperature, 260° C.), the remaining first metal component rate, thesolder runoff failure rate and the evaluation results thereof.

TABLE 3 Evaluation of remaining components Evaluation of RemainingEvaluation of Second bonding strength Evaluation of bonding first metalrunoff First metal metal (room temperature) strength (260° C.) componentRunoff component component Bonding Bonding rate failure (60% by (40% bystrength strength (% by rate volume) volume) (Nmm⁻²) Evaluation (Nmm⁻²)Evaluation volume) Evaluation (%) Evaluation Example Sn-3Ag-0.5Cu Cu-5Mn28 ⊙ 6 ◯ 19 ◯ 32 ◯ Cu-10Mn 27 ⊙ 24 ⊙ 0 ⊙ 0 ⊙ Cu-15Mn 28 ⊙ 25 ⊙ 0 ⊙ 0 ⊙Cu-20Mn 30 ⊙ 12 ◯ 9 ◯ 15 ◯ Cu-30Mn 31 ⊙ 5 ◯ 21 ◯ 35 ◯ Cu-5Ni 28 ⊙ 8 ◯ 12◯ 26 ◯ Cu-10Ni 30 ⊙ 26 ⊙ 0 ⊙ 0 ⊙ Cu-15Ni 29 ⊙ 26 ⊙ 0 ⊙ 0 ⊙ Cu-20Ni 30 ⊙12 ◯ 5 ◯ 12 ◯ Comparative Cu 31 ⊙ 0.1 X 31 X 75 X Example

As shown in Table 3, for the bonding strength at room temperature, bothExamples and Comparative Examples showed a bonding strength of 20 Nmm⁻²or greater and were found to have a practical strength.

On the other hand, for the bonding strength at 260° C., ComparativeExamples had an insufficient bonding strength of 0.1 Nmm⁻², far below 2Nmm⁻², while Examples retained a bonding strength of 5 to 26 Nmm⁻²,greater than 2 Nmm⁻², and were found to have a practical strength.Particularly, Examples showed a high bonding strength of 24 to 26 Nmm⁻²and were found to have excellent strength in high temperature when thesecond metal was Cu-10 to 15Mn and when the second metal was Cu-10 to15Ni.

For the remaining first metal component rate, Comparative Example had arate greater than 30% by volume while Examples all had a rate of 30% byvolume or less, and Examples had a remaining first metal component rateof 0% by volume when the second metal was Cu-10 to 15Mn and when thesecond metal was Cu-10 to 15Ni. For the solder runoff failure rate,Comparative Example had a rate of 70% or greater while Examples all hada rate of 50% or less, and Examples had a solder runoff failure rate of0% and were found to have high heat resistance when the second metal wasCu-10 to 15Mn and when the second metal was Cu-10 to 15 Ni.

Example 4

A powder of Sn-3Ag-0.5Cu was prepared as the first metal powder. Theaverage particle size of the first metal powder was 25 μm.

Powders of Cu and a Cu-10Mn alloy were prepared as the second metalpowder. The average particle size of the second metal powder was 15 μm.The particle size of the second metal powder was changed so that thespecific surface area was 0.03 to 0.06 m²·g⁻¹.

As the flux, one having a compounding ratio of rosin: 74% by weight,diethylene glycol monobutyl ether: 22% by weight, triethanolamine: 2% byweight and hydrogenated castor oil: 2% by weight was prepared.

A solder paste was prepared by mixing the above-mentioned first metalpowder, second metal powder and flux.

For the compounding ratio of the flux, the ratio of the flux to theentire solder paste was 10% by weight.

The compounding ratio of the first metal powder and the second metalpowder was adjusted so that the volume ratio of the first metalpowder/second metal powder was 60/40 (i.e. second metal powder: 40% byvolume).

For the solder paste thus prepared, the bonding strength, the remainingfirst metal component rate and the solder runoff failure occurrence ratewere measured to evaluate the characteristics in the same manner as inExample 1.

Evaluation of the bonding strength and evaluation of the remaining firstmetal component rate and the solder runoff failure rate were carried outon the basis of the same criteria as in Example 2 described above.

Table 4 shows the bonding strength of each joined body (roomtemperature, 260° C.), the remaining first metal component rate, thesolder runoff failure rate and the evaluation results thereof.

TABLE 4 Evaluation of remaining components Evaluation of Evaluation ofRemaining Evaluation of First Second bonding strength bonding strengthfirst metal runoff metal metal Specific (room temperature) (260° C.)component Runoff component component surface Bonding Bonding ratefailure (60% by (40% by area strength strength (% by rate volume)volume) (m2 · g⁻¹) (Nmm⁻²) Evaluation (Nmm⁻²) Evaluation volume)Evaluation (%) Evaluation Example Sn-3Ag- Cu-10Mn 0.06 29 ⊙ 24 ⊙ 0 ⊙ 0 ⊙0.5Cu 0.05 31 ⊙ 21 ⊙ 0 ⊙ 0 ⊙ 0.04 25 ⊙ 16 ◯ 6 ◯ 10 ◯ 0.03 28 ⊙ 14 ◯ 9 ◯15 ◯ Comparative Cu 0.06 31 ⊙ 0.1 X 31 X 75 X Example

As shown in Table 4, for the bonding strength at room temperature, bothExamples and Comparative Example showed a bonding strength of 20 Nmm⁻²or greater and were found to have a practical strength.

On the other hand, for the bonding strength at 260° C., ComparativeExample had an insufficient bonding strength of 0.1 Nmm⁻², far below 2Nmm⁻², while Examples retained a bonding strength of 14 to 24 Nmm⁻²,greater than 2 Nmm⁻², and were found to have a practical strength.Further, Examples showed a bonding strength of 21 Nmm⁻² or greater andhad particularly high strength in high temperature when the specificsurface area of Cu-10Mn, i.e. the second metal, was 0.05 m²·g⁻¹ orgreater.

For the remaining first metal component rate, Comparative Example had arate greater than 30% by volume while Examples all had a rate of 30% byvolume or less, and Examples had a remaining first metal component rateof 0% by volume when the specific surface area of Cu-10Mn, i.e. thesecond metal, was 0.05 m²·g⁻¹ or greater. For the solder runoff failurerate, Comparative Example had a rate of 70% or greater while Examplesall had a rate of 50% or less, and Examples had a solder runoff failurerate of 0% and were found to have high heat resistance when the specificsurface area of Cu-10Mn, i.e. the second metal, was 0.05 m²·g⁻¹ orgreater.

Example 5

A solder paste was prepared by mixing a metal powder of

a mixture of a Sn-plated Cu-10Mn alloy and a Sn powder,

a mixture of a Sn-plated Cu-10Mn alloy, a Sn powder and a Cu-10Mn alloyor

a Sn-plated Cu-10Mn alloy alone,

with a flux.

Except for the case where the Sn-plated Cu-10Mn alloy alone was used,the compounding ratio of the first metal powder and the second metalpowder was adjusted so that the volume ratio of the first metalpowder/second metal powder was 60/40 (i.e. second metal powder: 40% byvolume).

However, for the Sn-plated Cu-10Mn alloy alone, the total ratio of theCu—Mn alloy (second metal) was 80%.

As the flux, one having a compounding ratio of rosin: 74% by weight,diethylene glycol monobutyl ether: 22% by weight, triethanolamine: 2% byweight and hydrogenated castor oil: 2% by weight was used.

For the compounding ratio of the flux, the ratio of the flux to theentire solder paste was 10% by weight.

For the solder paste thus prepared, the bonding strength, the remainingfirst metal component rate and the solder runoff failure occurrence ratewere measured to evaluate the characteristics in the same manner as inExample 1.

Evaluation of the bonding strength, the remaining first metal componentrate and the solder runoff failure rate was carried out on the basis ofthe same criteria as in Example 2 described above.

Table 5 shows the bonding strength of each joined body (roomtemperature, 260° C.), the remaining first metal component rate, thesolder runoff failure rate and the evaluation results thereof.

TABLE 5 Evaluation Powder Total Total of remaining consisting amountamount Evaluation Evaluation components of second (% by (% by of bondingof bonding Remaining Evaluation Powder metal Powder volume) volume)strength (room strength first metal of runoff consisting componentconsisting of first of second temperature) (260° C.) component Runoff offirst coated with of second metal metal Bonding Bonding rate failuremetal first metal metal compo- compo- strength Eval- strength Eval- (%by Eval- rate Eval- component component component nent nent (Nmm⁻²)uation (Nmm⁻²) uation volume) uation (%) uation Example Sn Cn-coated —60 40 27 ⊙ 24 ⊙ 0 ⊙ 0 ⊙ Cu-10Mn Sn Cn-coated Cu-10Mn 29 ⊙ 24 ⊙ 0 ⊙ 0 ⊙Cu-10Mn — Cn-coated — 20 80 27 ⊙ 26 ⊙ 0 ⊙ 0 ⊙ Cu-10Mn Compar- SnSn-coated — 60 40 28 ⊙ 0.1 X 31 X 71 X ative Cu Example Sn Sn-coated Cu25 ⊙ 0.1 X 39 X 77 X Cu — Sn-coated — 20 80 28 ⊙ 0.1 X 43 X 84 X Cu

As shown in Table 5, for the bonding strength at room temperature, bothExamples and Comparative Examples showed a bonding strength of 20 Nmm⁻²or greater and were found to have a practical strength.

On the other hand, for the bonding strength at 260° C., ComparativeExamples had an insufficient bonding strength of 0.1 Nmm⁻², far below 2Nmm⁻², while Examples retained a bonding strength of 24 to 26 Nmm⁻²,greater than 2 Nmm⁻², and were found to have a practical strength.Hence, it was found that high strength in high temperature was obtainedas in the case of Examples described above even when the first metal wasplated (coated) on the surface of the second metal.

For the remaining first metal component rate, Comparative Examples had arate greater than 30% by volume, while Examples all had a rate of 0% byvolume. For the solder runoff failure rate, Comparative Examples had arate of 70% or greater while Examples all had a rate of 0%, and Exampleswere found to have high heat resistance even when the first metal wasplated (coated) on the surface of the second metal.

Example 6

A powder of Sn-3Ag-0.5Cu was prepared as the first metal powder. Theaverage particle size of the first metal powder was 25 μm.

A powder of a Cu-10Mn alloy was prepared as the second metal powder. Theaverage particle size of the second metal powder was 15 μm.

As the flux, a flux with a resin added thereto and a flux with no resinadded thereto were prepared.

As the flux with no resin added thereto, general flux A having acompounding ratio of rosin: 74% by weight, diethylene glycol monobutylether: 22% by weight, triethanolamine: 2% by weight and hydrogenatedcastor oil: 2% by weight was prepared.

For the flux with a resin added thereto, thermosetting resin-compoundedflux B with a thermosetting resin and a curing agent added to thegeneral flux A and thermoplastic resin-compounded flux C with athermoplastic resin added to the general flux A were prepared.

Thermosetting resin-compounded flux B contains the flux A, thethermosetting resin (bisphenol A type epoxy resin) and the curing agentat the ratio described below.

Flux A: 30% by weight

Thermosetting resin: 40% by weight

Curing agent: 30% by weight

Thermoplastic resin-compounded flux C contains the flux A and thethermoplastic resin (polyamide resin) at the ratio described below.

Flux A: 30% by weight

Thermoplastic resin (polyamide resin): 70% by weight

Then, following solder pastes were prepared:

a solder paste incorporating the flux A with no resin added thereto atsuch a ratio that the ratio of the flux to the entire solder paste is10% by weight;

a solder paste incorporating thermosetting resin-compounded flux B atsuch a ratio that the ratio of the flux to the entire solder paste is25% by weight; and

a solder paste incorporating thermoplastic resin-compounded flux C atsuch a ratio that the ratio of the flux to the entire solder paste is25% by weight.

For these solder pastes, the bonding strength, the remaining first metalcomponent rate and the solder runoff failure occurrence rate weremeasured to evaluate the characteristics in the same manner as inExample 1.

Table 6 shows the bonding strength of each joined body (roomtemperature, 260° C.), the remaining first metal component rate, thesolder runoff failure rate and the evaluation results thereof.

TABLE 6 Evaluation of remaining components Evaluation of Evaluation ofRemaining Evaluation of First Second bonding strength bonding strengthfirst metal runoff metal metal Presence/ (room temperature) (260° C.)component Runoff component component absence of Bonding Bonding ratefailure (60% by (40% by resin in strength strength (% by rate volume)volume) flux (Nmm⁻²) Evaluation (Nmm⁻²) Evaluation volume) Evaluation(%) Evaluation Example Sn-3Ag- Cu-10Mn Present 35 ⊙ 33 ⊙ 0 ⊙ 0 ⊙ 0.5Cu(bisphenol A epoxy resin) Present 32 ⊙ 30 ⊙ 0 ⊙ 0 ⊙ (polyamide resin)Absent 28 ⊙ 24 ⊙ 0 ⊙ 0 ⊙ Comparative Sn-3Ag- Cu — 25 ⊙ 0.1 X 31 X 75 XExample 0.5Cu

As shown in Table 6, for the bonding strength at room temperature, bothExamples and Comparative Example showed a bonding strength of 20 Nmm⁻²or greater and were found to have a practical strength.

On the other hand, for the bonding strength at 260° C., ComparativeExample had an insufficient bonding strength of 0.1 Nmm⁻², far below 2Nmm⁻², while Examples retained a bonding strength of 24 to 33 Nmm⁻²,greater than 2 Nmm⁻², and were found to have a practical strength.

For the remaining first metal component rate, Comparative Example had arate greater than 30% by volume, while Examples all had a rate of 0% byvolume. For the solder runoff failure rate, Comparative Examples had arate of 70% or greater, while Examples all had a rate of 0%, and werefound to have high heat resistance even when a resin was added.

In Examples described above, the present invention has been describedtaking as an example the case where the lattice constant of theintermetallic compound is greater than the lattice constant of thesecond metal, but theoretically, the present invention can also beconstituted such that the lattice constant of the second metal isgreater than the lattice constant of the intermetallic compound. In thiscase, by ensuring that the lattice constant difference is 50% orgreater, diffusion of the first metal and the second metal rapidlyproceeds, their change into an intermetallic compound having a highermelting point is facilitated, and almost no first metal componentremains, thus making it possible to solder with increased strength inhigh temperature.

The present invention is not limited to Examples described above, andvarious modifications and changes can be made within the scope of theinvention as to the type and composition of the first metal and thesecond metal constituting the solder paste, the compounding ratio of thefirst metal and the second metal, the components of the flux and thecompounding ratio of the flux, and so on.

In addition, various modifications and changes can be made within thescope of the invention as to the type of an object to be joined usingthe present invention, conditions in the joining step, and so on.

In other respects, various modifications and changes can be made as wellwithin the scope of the invention.

DESCRIPTION OF REFERENCE SYMBOLS

-   -   1 first metal    -   2 second metal    -   3 intermetallic compound    -   4 joint    -   11 a, 11 b a pair of electrodes (object to be joined)    -   10 solder paste

The invention claimed is:
 1. A solder paste comprising: a metal component comprising a first metal, a second metal having a melting point higher than that of the first metal, and a flux component, wherein the first metal is one of Sn and an alloy containing Sn, the second metal is one of (1) a Cu—Mn alloy in which a ratio of Mn to the second metal is 5 to 30% by weight and (2) a Cu—Ni alloy in which a ratio of Ni to the second metal is 5 to 20% by weight, and a ratio of the second metal to the metal component is 36.9% by volume or greater.
 2. The solder paste according to claim 1, wherein the first metal is one of Sn alone and an alloy containing at least one material selected from the group consisting of Cu, Ni, Ag, Au, Sb, Zn, Bi, In, Ge, Al, Co, Mn, Fe, Cr, Mg, Pd, Si, Sr, Te, P and Sn.
 3. The solder paste according to claim 1, wherein the second metal has a specific surface area of 0.05 m²·g⁻¹ or greater.
 4. The solder paste according to claim 1, wherein at least a portion of the first metal is coated on a circumference of the second metal.
 5. The solder paste according to claim 1, wherein the flux contains at least one material selected from the group consisting of: (a) at least one rosin material selected from a rosin group consisting of a rosin, a polymerized rosin, a water white rosin and a hydrogenated rosin; (b) a rosin-based resin containing a derivative of at least one material selected from the rosin group; and (c) a pasty matter containing a dissolved solid component and at least one solvent selected from the group consisting of ethylene glycol monobutyl ether, diethylene glycol monoethyl ether and diethylene glycol monobutyl ether.
 6. The solder paste according to claim 5, wherein the dissolved solid component is a thixotropic agent or an activator.
 7. The solder paste according to claim 1, wherein the flux contains one of (1) at least one material selected from a thermosetting resin group consisting of an epoxy resin, a phenol resin, a polyimide resin, a silicon resin, a derivative of the silicon resin, and an acryl resin, and (2) at least one material selected from a thermoplastic resin group consisting of a polyamide resin, a polystyrene resin, a polymethacryl resin, a polycarbonate resin and a cellulose resin.
 8. The solder paste according to claim 1, wherein a concentration of oxygen in the first metal and the second metal is 2000 ppm or less.
 9. The solder paste according to claim 8, wherein a concentration of oxygen in the first metal and the second metal is 10 to 1000 ppm.
 10. A method of joining an object, the method comprising: applying the solder paste according to claim 1 to the object to be joined; and forming the first metal component into an intermetallic compound with the second metal by heating to join the object to be joined.
 11. A joined structure comprising: an object; a solder joint in contact with the object, the solder joint being derived from the solder paste according to claim 1 and containing the second metal and an intermetallic compound containing the second metal and Sn, and represents 30% by volume or less of an entire metal component of the first metal.
 12. The joined structure according to claim 11, wherein the intermetallic compound was formed between one of (1) the Cu—Mn alloy and the Cu—Ni alloy and (2) one of Sn alone and an alloy containing at least one material selected from the group consisting of Cu, Ni, Ag, Au, Sb, Zn, Bi, In, Ge, Al, Co, Mn, Fe, Cr, Mg, Pd, Si, Sr, Te and P and Sn. 